Allow to override build user

This is meant to be a merge-request, but this gitlab would not let me fork because I "reached my project limit" at 0. You can use git am to get this commit:

From 0c579753b72bec6f5d185bf6d418e5d365f3b86d Mon Sep 17 00:00:00 2001
From: "Bernhard M. Wiedemann" <bwiedemann@suse.de>   
Date: Sat, 8 Aug 2020 13:51:53 +0200
Subject: [PATCH] Allow to override build user

similar to commit 245d5a3c39cf69e9ecb10894addb69daea6ce0e9

See https://reproducible-builds.org/ for why this is good.

This PR was done while working on reproducible builds for openSUSE.
---
 CMakeLists.txt | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt

index 5ac523e..68c5295 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -119,10 +119,12 @@ if(NOT GETDP_HOST)
   set(GETDP_HOST "${HOSTNAME}")
 endif()
 
-if(NOT PACKAGER)
-  set(PACKAGER "unknown")
+if(NOT GETDP_PACKAGER)
+  if(NOT PACKAGER)
+    set(PACKAGER "unknown")
+  endif()
+  set(GETDP_PACKAGER "${PACKAGER}")
 endif()
-set(GETDP_PACKAGER "${PACKAGER}")
 
 if(APPLE)
   set(GETDP_OS "MacOSX")
-- 
2.26.2